The invention relates to a tap whose threaded portion forms a cutting section and a guiding section and is essentially cylindrical and provided on a plurality of adjacent thread teeth of the cutting section with a relief whose clearance angle is related to the thread tooth with the maximum pitch or effective diameter.
In a prior art (DE-PS 32 26 355) tap of this kind, the reliefs of the adjacent cutting-section thread teeth--seen in a longitudinal direction--have constant clearance angles. If the relief is kept small, the guiding properties are little affected and friction or the risk of cold welding is relatively high. If the relief is large, the guiding properties would be appreciably affected and friction or the risk of cold welding would be relatively small. With a view to better reconciling the contradictory requirements with respect to guiding properties and friction, the tap is formed with a back taper over the thread section. However, the magnitude of the back taper must be only small because otherwise the thread produced may have profile errors.